Pulsed-discharge-nozzle Fourier-transform microwave spectroscopy of HC3S(2Πr) and HC4S(2Πi)

Abstract

Rotational spectra of sulfur‐bearing carbon‐chain free radicals HC₃S and HC₄S have been observed for the first time by using a Fabry–Perot type Fourier‐transform microwave spectrometer combined with a pulsed discharge nozzle. Rotationally cooled (∼1–2 K) HC₃S(²Π_1/2) and HC₄S(²Π_3/2) in their ground vibronic states have been produced by a discharge in a supersonic free jet of an Ar‐diluted mixtures of C₂H₂ and CS₂. Transitions of an isotopic species, HC₃³⁴S, have also been observed in the same system, and those of DC₃S and DC₄S have been observed by using C₂D₂ instead of C₂H₂. The observed spectral patterns for these species have clearly indicated that HC₃S and HC₄S are linear free radicals in the ground electronic states with ²Π symmetry. The spectra of HC₃S, HC₃³⁴S, and DC₃S have shown both the Λ‐type doubling and the hyperfine splittings due to the H or D nucleus, whereas those of HC₄S and DC₄S have shown only the hyperfine splittings. The observed transition frequencies for the radicals in the 7–25 GHz region have been fitted to the Hamiltonian appropriate to ²Π electronic states, yielding the effective molecular constants including the hyperfine parameters. The total bond length of HC₃S has been determined to be 5.107 Å from the substituted‐coordinate analysis.